Re: [radext] Shepherd review of draft-ietf-radext-tls-psk-03

Fabian Mauchle <fabian.mauchle@switch.ch> Tue, 14 November 2023 10:40 UTC

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References: <091b01da0cd0$570fb8f0$052f2ad0$@smyslov.net> <F75CF375-73B2-43C9-A047-6350FA2DFD8A@deployingradius.com> <09ca01da0d60$573d2f20$05b78d60$@smyslov.net> <CAA7Lko-r8pGYoewJwj-zPuy+ZGTn9_wPzusumyG_MdN50QKD5w@mail.gmail.com> <02d601da10a8$50f9a8a0$f2ecf9e0$@gmail.com>
From: Fabian Mauchle <fabian.mauchle@switch.ch>
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Subject: Re: [radext] Shepherd review of draft-ietf-radext-tls-psk-03
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Hi all,

On 06.11.23 12:56, Valery Smyslov wrote:
> With TLS 1.3 a server can choose to not do PSK authentication and 
> continue the handshake with certificate authentication. If this is 
> desired, then 'Any connection from outside of the allowed range(s) MUST 
> be rejected' wouldn't work if certificate authentication for the non-PSK 
> address range is supported. Or if 'rejected' in this context simply 
> means that PSK is not used, then a clarification would be useful.
> 
>              My understanding is that “MUST be rejected” here applies to 
> PSK cases only.
> 
>              If the server falls back to certificates, then 
> “RECOMMENDED” below works.

 From the extensive discussions at the IETF118 we concluded that a 
server should reject a connection if it doesn't recognize the PSK 
identity as a configured client or as a resumption ticket; for the PSK 
identity is part of the server authentication.

However, looking at the TLS spec (RFC8446), it does not mandate any 
mechanism how to create the session tickets and thus there is no 
guarantee that an implementation will be able to distinguish between 
externally established PSK and session resumption.
I'm therefore reluctant to put an unconditional MUST on this.

My proposal therefore would be:

5.  Guidance for RADIUS Clients

(add to the end of the section, before 5.1)
+ If a client initiated a connection using a pre-shared key with TLS1.3
+ by inlcuding the pre-shared key extension, it MUST reject the
+ conneciton if the server did not select the pre-shared key to continue
+ the handshake.

6.2.1.  Requirements for TLS-PSK

   Finally, if a RADIUS server does not recognize the PSK identity or if
   the identity is not permitted to use PSK, then the server
+ MUST reject the connection unless it is configured for session
+ resumption. Since TLS 1.3 [RFC8446] uses PSK for resumption, a server
+ SHOULD colorize the session ticket and SHOULD only perform a full
+ handshake if the provided PSK is identified as a (possibly outdated or
+ invalid) session ticket the server handed out.


BR,
Fabian

-- 
Fabian Mauchle
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